The image shows Grimes' face superimposed on the body of "Obama Girl," a model who made racy videos about her crush on then-Sen. Barack Obama in 2007. The woman is wearing a snug-fitting Obama T-shirt with her midriff showing.

"We agree; it's extremely offensive," said NRSC spokeswoman Brook Hougesen. "It was a mistake made by a junior staffer and disciplinary action has been taken. We took corrective action as soon as it was brought to our attention and have taken steps to ensure it will never happen again."

Grimes is the Democratic front-runner for her party's nomination to take on U.S. Senate Republican Leader Mitch McConnell. McConnell also faces a GOP primary challenger, Louisville businessman Matt Bevin.

In a statement, Grimes condemned the tweet as "sexist."

"As I have said, I am proud to wear a dress," Grimes said. "And as Kentucky's more than two million women know, it is not what is in the dress that matters. It is what is in the head, and I will stack my head up against Sen. McConnell's any day."

The McConnell campaign voiced no sympathy for Grimes in a statement Tuesday afternoon. McConnell spokeswoman Allison Moore instead accused Grimes of trying to use the incident to distract from "the real news" that she attended a fundraiser with first lady Michelle Obama in New York on Monday.

"It is really pathetic that the Grimes campaign is more worried about what some intern in the basement of the NRSC is tweeting rather than talking about the real issues facing Kentucky, but I am sure we will see this call from their playbook time and time again," Moore said. "Mitch McConnell is dedicated to running on the issues and challenges Alison Grimes to do the same."

McConnell, a five-term incumbent, has made Barack Obama a focal point of his re-election campaign, trying to paint Grimes as a rubber stamp for a president who is widely unpopular in Kentucky.

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